1. A Field of the Invention:
This invention relates to internal mixers. More particularly, it relates to the shape of the profile of the blades with which these rotors are equipped.
2. Description of the Related Art:
Internal mixers are mixers of discontinuous operation suitable for the preparation of rubber and other plastic materials. They comprise a closed tank with one or two cylindrical chambers which are connected to one another, each chamber comprising a rotor. Each rotor has one or more blades. The rotor is characterized by its profile (section by a plane perpendicular to the axis of rotation defining the profile of the blades) and by its diagram which is a flat development on a plane parallel to the rotation axis which gives the positioning of the blades at the surface of the rotor, particularly the position, the orientation relative to the edges, and the extent of the zone of minimum space between the wall of the chamber and the surface of the rotor (zone subsequently called "fine passage").
The choice and the combination of the number of chambers as well as the type of rotor used depends on the composition of the mixtures of rubber or plastic materials to be made. The most widely used internal mixers for the preparation of unvulcanized rubber comprise two chambers and two nonmeshing rotors, turning in opposite directions at various rates. An example is seen in U.S. Pat. No. 4,456,381.
The object sought in internal mixers is to force the components of the mixture to pass from one chamber to another and, inside one chamber, from one side to another, in the axial direction of the latter, and also to force the mixture to pass between the blades of the rotors and the walls of the tank. This makes it possible to make a macrodispersion of the components and a microdispersion of the additives. The macrodispersion involves the transfer of mass of material between the chambers of the tank and the axial transfers, i.e. along the blades of the rotor in each chamber, whereas the microdispersion is primarily provided by the passage between the blade and the wall of the chamber.
The rotors of internal mixers have the drawback of exhibiting, in the fine passage part, a zone of intense work of short length, which stresses the materials in a sudden and irregular manner and causes significant slippages, as well for certain elastomers cohesion losses accompanied by a poor mixing state. FIG. 1 is a view of a known blade profile 10 of rotor 1. In particular, the presence of sharp edges 20 which delimit, on both sides, fine passage zone 21 is seen. The side defined as being in front of fine passage 21 that upstream from the latter in the direction of rotation of rotor 1 (arrow 11), which therefore corresponds to a convergent surface of the blade cooperating with the wall of chamber 2. Conversely, the back side has a divergent surface and is located on the opposite side from the convergent surface relative to the direction of rotation. Behind fine passage 21, it is also seen that there is a sudden and significant increase of the space between blade 10 and the wall of chamber 2.
In addition, conventional internal mixers cannot assure a good control of the heat exchanges which occur during the mixing cycle.